The present invention relates generally to digital communication systems, and more particularly, to a digital communication system employing paired power amplifiers and drift compensation feedback control loops.
In digital communications the quest for ever increasing data rates in a limited bandwidth has led to types of wideband vector modulation of an RF carrier, such as M-ary amplitude-phase shift-key (APK) modulation of an RF carrier, which are very sensitive to distortion.
In its conceptually simplest form the wideband vector modulation would incorporate a pair of very high throughput digital to analog converters, a quadrature modulator and a power amplifier to drive an antenna. Unfortunately microwave high power amplifiers suffer from AM to PM and AM to AM distortion, especially when operated near saturation, where they are most efficient. The effect is especially severe for TWT amplifiers, but all types of amplifiers are affected.
Paired power amplifiers achieve both high efficiency and low distortion by using two amplifiers, which are driven by constant amplitude phase modulated signals. The concept of using paired power amplifiers achieves both high efficiency and low distortion. Two amplifiers are driven by constant amplitude phase modulated signals and the APK vector is formed by signal combining in the RF passband. The control loops, which are the subject of the present invention, are essential to the practical implementation of Paired Power Amplifiers.
For this technique to be effective, there has to be an exact match between the phase and amplitude characteristics of the two signal paths through the amplifiers. For this reason the drift compensation feedback control loops, which are the subject of the present invention, are essential to the practical implementation of paired power amplifiers. Prior art paired amplifier arrangements have not used drift compensation. Prior art paired power amplifiers is disclosed in U.S. Pat. No. 5,329,259 entitled "Efficient Amplitude/Phase Modulation Amplifier", for example.
Therefore, it would be advantageous to have a digital communication system comprising paired power amplifiers that use drift compensation feedback control loops to provide improved operating characteristics.